./trace-it 1 > trace.txtdoes it produce lots of trace entries? If not then CONFIG_FUNCTION_TRACING is not enabled. Once you see lots of output in the file, the tracer is up and running and you can start tracing the latency in your app.

your above wakeup-tracing output suggests that your app is probably not delayed by scheduling latencies, but by some other, higher-level latencies.

To track it down, use the method that trace-it.c uses to start/stop tracing, i.e. put the prctl(0,1); / prctl(0,0); calls into your app to start/stop tracing and the kernel will do the rest once you've set /proc/sys/kernel/preempt_max_latency back to 0: /proc/latency_trace will always contain the longest latency that your app triggered, of the critical path you programmed into it.

also check the cyclictest source of how to do app-driven latency tracing. (And please post any latency traces to this list, we might be able to pinpoint the source of the latencies.)